1. Field of the Invention
The present invention relates to a calibration disk that can be used for calibrating a glide head, used in the process of manufacturing hard memory disks.
2. Description of the Related Art
The key components of a hard disk drive are a magnetic disk and a magnetic head, which is typically separated from the magnetic disk by a small gap. The gap is created by the magnetic head remaining relatively stationary while the magnetic disk rotates on a spindle. The rotation of the magnetic disk generates a thin film of air known as an xe2x80x9cair bearingxe2x80x9d over the surface of the disk that supports the magnetic head, which is essentially flying over the surface of the magnetic disk.
The permitted recording density of the magnetic disk is strongly influenced by the gap or xe2x80x9cfly heightxe2x80x9d between the disk and the magnetic head. Because decreasing the fly height of the magnetic head allows the recording density of the magnetic disk to be increased, the magnetic disk should have a very smooth surface so the magnetic head can fly very close to the surface. Magnetic disks with protrusions related to defects or contamination that exceed the fly height of the magnetic head should not be used in production hard disk drives. This is because an impact between the magnetic head and a protrusion can cause undesirable effects, such as a hard drive crash, formation of wear debris, unusable recording area, a thermal spike in a magnetoresistive head and the like. In order to ensure acceptable surface conditions of the magnetic disk, glide tests are widely employed by the hard disk industry for purposes of quality control.
The basic operation of the glide test is to fly a test head, i.e., a glide head, at a height related to the fly height and margin requirements of the magnetic head, and to sense any contact between the glide head and any defects on the surface of the magnetic disk. If the glide head contacts a defect, the disk is rejected. The term xe2x80x9cglide headxe2x80x9d as used herein indicates a head used in a magnetic disk testing system, as distinguished from the term xe2x80x9cmagnetic head,xe2x80x9d which is used in general to refer to a read-write head.
The contact detection is typically accomplished with a piezoelectric (PZT) sensor or an acoustic emission (AE) sensor. The PZT sensor, as is well known to those skilled in the art, uses a piezoelectric crystal to convert mechanical energy into an electrical signal. Thus, a PZT sensor converts the mechanical energy generated by the glide head contacting a defect into an electrical signal that can be used to indicate the size and location of the defect.
An AE sensor uses a sensing technique similar to a PZT sensor. The difference is the mounting position and configuration of the sensor. The sensing material in an AE sensor is typically a PZT (PbZrO3-PbTiO3) ceramic that has a piezoelectric effect and which is housed in a metal container and mounted close to the head/slider suspension. Both the PZT and AE sensors give electrical signals excited by acoustic vibration. For more information related to the aforementioned sensing technology see U.S. Pat. Nos. 5,423,207 and 4,532,802.
The magnetic disk can also be tested for defects using non-contact methods such as a magneto-resistive (MR) head, a laser, or an optical tester. For more information related to MR technology see U.S. Pat. No. 5,527,110, and see U.S. Pat. No 5,550,696 for a method to calculate a protrusion height based on a diffracted laser beam detected by a linear photo-detector array. An optical tester optically scans the magnetic disk for defects. The detection is usually performed by comparing the light reflected from a defect with the light reflected from an area of the disk that does not have defects. The optical tester is calibrated in such a way that a rejection of a magnetic disk occurs when the height of a defect is above a desired threshold.
Another important parameter of the magnetic disk is the minimum height at which a head can fly without contacting the disk surface, known as the avalanche height, which occurs at the avalanche point on an avalanche curve (described subsequently). It will be noted that the avalanche height can differ from the fly height of the glide head. While the fly height is usually determined by potential extrinsic defects, such as contamination, the avalanche height is determined by intrinsic surface topology. The avalanche height is defined as the fly height at which the lowest part of the head starts to contact the disk surface. For example, the landing zone of a magnetic disk, which is usually heavily textured to prevent excessive friction, has a relatively large avalanche height due to the additional surface roughness created by the heavy texturing. The data zone, however, has a smoother surface because there is no need to reduce friction. Consequently, a glide head can fly lower over the data zone than the landing zone, and thus, the data zone has a relatively lower avalanche height. The avalanche height is a useful indication of the surface finish and gives an absolute fly height below which flying is not possible without contacting the disk.
While magnetic disks are ideally flat and smooth, in practice there is typically an amount of disk waviness and runout. Disk waviness causes the effective height of a disk""s surface to vary relative to the mean disk surface. If the waviness of the disk surface has a wavelength that is less than a longitudinal dimension of the glide head, the glide head cannot follow the disk surface. Consequently, the amplitude of the waviness needs to be accounted for when determining fly height. A typical amplitude of the waviness of a disk is 20 to 60 nm (nanometers) and has a wavelength that is typically smaller than the length of a conventional glide head.
Disk runout is a deviation from a level surface and is caused by improper clamping of the disk, for example. The runout effect typically creates a variation from a level surface over an area of the disk that is much greater than the length of the glide head. Disk runout causes acceleration of the glide head which induces fly height fluctuations. A typical disk runout is approximately 2 to 10 xcexcm (micrometers).
To accurately test a magnetic disk with a glide head, it is important for the glide head to be calibrated so that the fly height at which the test is carried out is known. Calibration ensures that the threshold for defects is set to an appropriate limit (i.e., the height above which a surface defect becomes unacceptable). A conventional method of calibration is performed by flying a glide head over a glass disk on a fly height tester. The fly height tester operates by passing a beam of light through the glass disk. The interference pattern of light reflected off the glide head and light reflected off the surface of the glass disk is used to determine the distance between the disk surface and the glide head. This procedure is performed for a number of different linear velocities of the glass disk to establish the relationship between linear velocity and fly height for that particular glide head.
The linear velocity versus fly height relationship is then used to determine the linear velocity at which to fly the glide head over production disks on a glide tester. Thus, a linear velocity can be selected that achieves the desired glide height, in order to test for defects on a production disk that are higher than the glide height. This procedure is performed for each individual glide head because each glide head has different flying characteristics.
There are several drawbacks to the use of a fly height tester for calibration of a glide head. For example, the fly height tester uses a glass disk, which may have different characteristics than a production memory disk, including differences in waviness, runout and the like. Changes in surface topology will cause a change in the flying characteristics of the glide head and, thus, the fly height of the glide head may be different when the glide head flies over a production disk. The difference in waviness and runout between a glass disk and a production magnetic disk can also affect calibration accuracy.
In addition, the fly height tester measures the fly height of the glide head at a limited number of points, i.e., only at the points where the light is incident on the glide head. Because the attitude of a glide head in flight is not flat (i.e., the leading edge of the glide head is flying higher than the trailing edge), portions of the glide head may actually be lower than the points on the glide head being illuminated. Thus, the actual fly height of the lowest point on the glide head may be lower than the measured fly height.
Further problems encountered with using a fly height tester for calibration arise from the fact that the glide head is first installed on the fly height tester to determine fly height. The glide head is then removed from the fly height tester and installed on the glide tester to test production disks. Installing the glide head on two separate systems is time consuming and thus results in a loss of productivity. In addition, the glide head may be installed slightly differently on the glide tester than on the fly height tester. Differences in the installation on the two devices can cause differences in the flying characteristics of the glide head, including skew and mount flatness, thereby resulting in an inaccurate calibration of the glide head. Moreover, the Z height, which is the height between the disk surface and the suspension arm upon which the glide head is mounted, may differ between the fly height tester and the glide tester. Variances in Z height can also adversely affect the repeatability of the flying characteristics of the glide head from the fly height tester to the glide tester.
In accordance with the present invention, a calibration disk includes calibration areas that allow a glide head to be calibrated as to the glide head""s avalanche height, fly height and the like. The calibration areas may be, for example, configured in a circumferential band (or, alternatively, a series of circumferential bands extending substantially concentric to one another), one or more spiral bands or some other suitable configuration. Such bands can, for example, extend from adjacent an outer diameter of the calibration disk to adjacent an inner portion of the calibration disk, and are typically circular in shape (and are therefore also referred to herein as circular bands). Each of the circumferential bands is a textured area on the calibration disk having a given degree of composite roughness, which can be measured, for example, by the given circumferential band""s average composite roughness height, and as such is referred to herein as a calibration band. This texturing can be by mechanical zone texturing, laser zone texturing or any other method that produces the desired composite roughness. Preferably, the given calibration band is textured in a uniform manner, sufficient to produce a constant and continuous output signal from a sensor mounted on a glide head when the glide head flies at an avalanche height. Also preferably, the average composite roughness height within an individual calibration band is substantially uniform, and the average composite roughness height of each calibration band is differs from that of the other calibration bands. As used herein the term xe2x80x9ccalibration bandsxe2x80x9d is not to be confused with data tracks typically found on a production read-write magnetic hard disk.
The calibration disk can be, for example, double-sided, with calibration bands on both sides of the disk. Further, such a calibration disk can be constructed, for example, from a material such as nickel-phosphorus-plated aluminum, and can also have magnetizable layers on its surface. The calibration bands can be produced using, for example, mechanical zone texturing or laser zone texturing in a concentric or spiral pattern, or in any other appropriate manner. Average composite roughness height is determined by measuring composite roughness height at various points in a calibration band and then using the statistical formulae described herein to arrive at an estimation of the average composite roughness height for a given area on the calibration disk in question. In addition, such a technique compensates for the waviness and runout effects of a disk as well as variations of fly height of glide heads.
A calibration disk according to the present invention can be used to check the actual flying conditions of a glide head while the glide head is mounted on a glide tester system, thereby obviating the need to independently test the fly height of the glide head on a fly height tester. Consequently, productivity loss and other problems associated with a fly height tester, such as variance in the glide head""s flight characteristics due to remounting the glide head on the glide tester, are avoided.
In one embodiment, a calibration disk for calibrating a head is described. The calibration disk includes a calibration area created by texturing and having a given composite roughness. This composite roughness can be created, for example, by mechanical zone texturing or laser zone texturing. The calibration area can be, for example, in the form of a circumferential band. The width of such a circumferential band is preferably greater than a width of the head. The circumferential band can be one of a number of such circumferential bands extending substantially concentric to one another, where each of the circumferential bands has a composite roughness. The composite roughness can be described in terms of a given micro-waviness and a given roughness. The maximum wavelength of the micro-waviness can be defined as less than about a length of said head, while a minimum wavelength of said micro-waviness can be defined as being greater than about a feature size of said roughness. The composite roughness can be described, for example, in terms of composite roughness height or average composite roughness height.
In another embodiment, a method of calibrating a glide head with a calibration disk is described. The method begins with providing a calibration disk having a calibration area. The calibration area exhibits a composite roughness, which can be described, for example, in terms of composite roughness height or average composite roughness height. Next, a glide head is flown over the calibration area. While flying the glide head over the calibration area, a substantially constant signal indicative of contact between said glide head and said calibration area is detected. A recorded linear velocity at which said signal is detected is recorded, and a test linear velocity based upon said recorded linear velocity is determined.
The foregoing is a summary and thus contains, by necessity, simplifications, generalizations and omissions of detail; consequently, those skilled in the art will appreciate that the summary is illustrative only and is not intended to be in any way limiting. Other aspects, inventive features, and advantages of the present invention, as defined solely by the claims, will become apparent in the non-limiting detailed description set forth below.